Device and a method of cleaning an effluent gas from an aluminium production electrolytic cell

ABSTRACT

A gas cleaning unit for cleaning an effluent gas of at least one aluminium production electrolytic cell comprises a contact reactor in which the effluent gas is brought into contact with alumina, and a dust removal device for removing at least a portion of the alumina. The gas cleaning unit further comprises a wet scrubber in which the effluent gas is brought into contact with an absorption liquid containing water for removing further pollutants from the effluent gas. The wet scrubber is positioned at a point vertically higher than that of the dust removal device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional of U.S.application Ser. No. 13/968,703, filed on Aug. 16, 2013, which in turnclaims priority to PCT Application No. PCT/IB2012/000231, filed on Feb.8, 2012, which in turns claims priority to European Application No.11154939.0, filed Feb. 18, 2011, the contents of which are all herebyincorporated in their entirety.

FIELD OF THE INVENTION

The present invention relates to a gas cleaning unit for cleaning aneffluent gas of at least one aluminium production electrolytic cell, thegas cleaning unit comprising at least one contact reactor in which theeffluent gas is brought into contact with alumina, and a dust removaldevice in which at least a portion of the alumina having adsorbedpollutants from the effluent gas in the contact reactor is separatedfrom the effluent gas.

The present invention further relates to a method of cleaning aneffluent gas of at least one aluminium production electrolytic cell.

BACKGROUND

Aluminium may be produced by electrolytic reactions in aluminiumproduction electrolytic cells, sometimes called electrolytic smeltingpots, using the Hall-Héroult process. An example of an electrolyticsmelting pot is disclosed in US 2009/0159434.

The electrolytic reaction occurring in the electrolytic smelting potsproduces effluent gas in the form of hot, particle-laden effluent gas,which is cleaned in a gas cleaning unit before being discharged to theatmosphere. An example of a gas cleaning unit for cleaning the effluentgas generated in electrolytic smelting pots is disclosed in U.S. Pat.No. 5,885,539. The gas cleaning unit disclosed in U.S. Pat. No.5,885,539 comprises a first contact reactor and a second contactreactor. The effluent gas from the electrolytic smelting pots is firstforwarded to the first contact reactor and is, in the first contactreactor, brought into contact with recycled alumina. The partly cleanedeffluent gas is then forwarded to the second contact reactor and is, inthe second contact reactor, brought into contact with fresh alumina. Thepartly used alumina is recycled from the second contact reactor to thefirst contact reactor. A dust removal device removes the alumina fromthe effluent gas which is then discharged to the atmosphere.

SUMMARY

An object of the present invention is to provide a gas cleaning unit forremoving gaseous pollutants from an effluent gas of an aluminiumproduction electrolytic cell that is more efficient with respect toefficiency of removing pollutants from the effluent gas than that of theprior art.

The above-noted object is achieved by a gas cleaning unit for cleaningan effluent gas of at least one aluminium production electrolytic cell,the gas cleaning unit comprising at least one contact reactor in whichthe effluent gas is brought into contact with alumina, and a dustremoval device in which at least a portion of the alumina havingadsorbed pollutants from the effluent gas in the contact reactor isseparated from the effluent gas. The gas cleaning unit further comprisesa wet scrubber in which the effluent gas being forwarded from the dustremoval device is brought into contact with an absorption liquidcontaining water for removing further pollutants from the effluent gas,the wet scrubber being arranged on a higher vertical level than the dustremoval device.

An advantage of the above-described gas cleaning unit is that a veryefficient removal of pollutants, such as sulphur dioxide, hydrogenfluoride, and dust particles, from the effluent gas is obtained, bothwith regard to the removal efficiency as such, and with regard to theinvestment and operating costs of the gas cleaning unit. A very compactgas cleaning unit may be obtained, with minimal duct work required, andminimum effluent gas transport distances. The relatively short effluentgas transport distances result in relatively low gas cleaning unitpressure drop and energy consumption.

According to one embodiment the wet scrubber portion of the subject gascleaning unit is positioned at a point vertically higher than and besidethe dust removal device. An advantage of this embodiment is that a verycompact gas cleaning unit is obtained, without sacrificing service andmaintenance of the dust removal device.

According to one embodiment of the subject gas cleaning unit, an aluminasilo is arranged beside the dust removal device with the wet scrubberpositioned over at least a portion of the alumina silo and at leastpartly obscuring the alumina silo when the wet scrubber is viewed fromabove. An advantage of this embodiment is that the silo is efficientlyintegrated in the gas cleaning unit with the wet scrubber positionedabove the alumina silo, which infrequently requires any maintenancework.

According to one embodiment the dust removal device comprises a cleangas plenum arranged at the top thereof. The wet scrubber device isfluidly connected to the clean gas plenum via an outlet duct arranged ina side wall of the clean gas plenum. An advantage of this embodiment isthat a combination of good serviceability with regard to the dustremoval device and compact arrangement with regard to short transportdistances for the effluent gas, results in low gas cleaning unit energyconsumption.

According to one embodiment a fan is connected to an outlet duct of aclean gas plenum of the dust removal device to cause a flow of effluentgas from the clean gas plenum to the wet scrubber. An advantage of thisembodiment is that the fan is integrated for the flow of the effluentgas from the clean gas plenum to the wet scrubber. Hence, no, or onlylimited, space is required for the fan, and a very compact arrangementis obtained.

According to one embodiment a radial fan comprising an impeller rotatingon a horizontal shaft receives effluent gas flowing in a horizontaldirection from the clean gas plenum of the dust removal device andtransports the effluent gas upward into the wet scrubber positionedthereabove. An advantage of this embodiment is that the radial fanperforms the dual functions of forwarding the flow of effluent gas fromthe clean gas plenum of the dust removal device to the wet scrubber, andof diverting the flow of effluent gas from a horizontal flow to anupward vertical flow.

According to one embodiment the dust removal device and the wet scrubbertogether form a common stacked unit, and are supported on a commonsupport structure. An advantage of this embodiment is that the gascleaning unit is less complex, requires an overall smaller footprint andhas a lower investment cost, since the number of required supportstructures is minimized.

According to one embodiment a gas cleaning unit penthouse houses atleast a part of a clean gas plenum of the dust removal device, and atleast a part of the wet scrubber. An advantage of this embodiment isthat the wet scrubber and the clean gas plenum are protected from, forexample, wind loads, precipitation, sunlight, and sand storms. As such,requirements with regard to suitable equipment material types andmaterial dimensions may be lowered, thereby reducing required investmentcosts.

According to one embodiment a wet scrubber inlet opening for receivingeffluent gas flowing from the dust removal device is arranged in abottom of the wet scrubber. An advantage of this embodiment is that thewet scrubber can be arranged in very close proximity to the dust removaldevice, since positioned at a level vertically above the level of thedust removal device. Preferably, a gas distributor is arranged at thebottom of the wet scrubber for distributing effluent gas that enters thewet scrubber from below.

According to one embodiment a stack for discharging cleaned effluent gasis arranged on top of the wet scrubber. An advantage of this embodimentis that the wet scrubber also serves as a part of the stack.Furthermore, there is no need for long ducts to channel the flow ofcleaned effluent gas to a remotely arranged stack. As such, investment,operating and maintenance costs are reduced.

A further object of the present invention is to provide a method ofremoving gaseous pollutants from an effluent gas of an aluminiumproduction electrolytic cell that is more efficient with respect toremoving pollutants from the effluent gas than is the method of theprior art.

This object is achieved by means of a method comprising:

contacting effluent gas with alumina adsorbing at least a portion of thecontent of pollutants of the effluent gas;

separating at least a portion of adsorbed pollutants from the effluentgas using a dust removal device; and

contacting the effluent gas with an absorption liquid comprising waterin a wet scrubber positioned at a point vertically higher than that ofsaid dust removal device to further remove pollutants from the effluentgas.

An advantage of this method is that pollutants may be removed from theeffluent gas in an efficient manner with respect to investment andoperating costs, and with respect to the cleaned effluent gas puritylevel upon release from the gas cleaning unit.

According to one method embodiment, the effluent gas from which at leasta portion of the alumina has been separated is forwarded into a cleangas plenum arranged at the top of the dust removal device. The effluentgas flows horizontally out of the clean gas plenum before being divertedto flow vertically upward into the wet scrubber. An advantage of thisembodiment is that a compact and efficient arrangement is obtained,which still enables maintenance of the dust removal device.

According to one method embodiment, the effluent gas flows upwardlyand/or horizontally while being subjected to the steps of: contactingalumina, separating from the alumina, entering the wet scrubber, andcontacting absorption liquid in the wet scrubber, during which theeffluent gas flows upwardly in at least one of the steps. With the gasflowing upwardly, and optionally horizontally, for shorter distances,during the treatment steps, the effluent gas moves a relatively shorttotal distance. Additionally, the gas does not move downwardly to anysubstantial extent during the treatment steps. Such reduces investmentand operating costs by minimizing gas duct lengths and required fanpower. Also, the method may be conducted in a gas cleaning unit having arelatively smaller overall footprint.

Further objects and features of the present invention will be apparentfrom the following detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with reference to theappended drawings in which:

FIG. 1 is a schematic side view, in cross-section, of a gas cleaningunit cleaning effluent gas from at least one aluminium productionelectrolytic cell;

FIG. 2 is a schematic side view of a gas cleaning unit, viewed in thedirection depicted by arrows II-II of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of a gas cleaning unit 1 incross-section viewed from the side thereof. The gas cleaning unit 1comprises, as its main components, a gas inlet duct 2, a first contactreactor 4, a second contact reactor 6, an alumina silo 8, a dust removaldevice 10, a wet scrubber 12, a gas cleaning unit penthouse 14, and astack 16. Arrows A indicate the intended flow path of the effluent gasthrough the gas cleaning unit 1.

FIG. 2 illustrates the gas cleaning unit 1 viewed in the direction ofarrows II-II of FIG. 1, i.e., as viewed from the side of FIG. 1. The gasinlet duct 2 is connected to a collecting duct 18, illustratedschematically and not drawn to scale, collecting effluent gas from eachof, typically, 1 to 400, more typically 5 to 200, aluminium productionelectrolytic cells 20, each of which may be operative for producingaluminium in accordance with, for example, the above mentionedHall-Héroult process.

Returning to FIG. 1, the gas inlet duct 2 channels the effluent gas flowfrom the aluminium production electrolytic cells to the first contactreactor 4. A volumetric feeder 22 is operative for recirculatingalumina, Al₂O₃, in the first contact reactor 4 to provide for efficientcontact between the alumina and the effluent gas. As an effect of suchcontact, gaseous pollutants, such as hydrogen fluoride, HF, and sulphurdioxide, SO₂, are adsorbed by the alumina.

The effluent gas flows further to the second contact reactor 6. A supplypipe 24 with a volumetric feeder 25 is arranged for supplying freshalumina to the second contact reactor 6 from the fluidly connectedalumina silo 8. The fresh alumina effects a further adsorption ofpollutants from the effluent gas on alumina. The alumina silo 8 isarranged beside the dust removal device 10, and share a common verticalside wall 11. The wet scrubber 12 is arranged above the alumina silo 8and at least partly obscures the view of alumina silo 8 when lookingvertically downward from the top of the wet scrubber 12, as is clearfrom the combined perspectives of FIG. 1 and FIG. 2.

The effluent gas, comprising particles entrained from the aluminiumproduction electrolytic cells 20, recycled alumina entrained from thefirst contact reactor 4, and fresh alumina entrained from the secondcontact reactor 6, flows to the dust removal device 10. The dust removaldevice 10 is arranged above the second contact reactor 6. The dustremoval device 10 may, for example, be an electrostatic precipitator,the basic principle of which is known from, for example, U.S. Pat. No.4,502,872, or a fabric filter, the basic principle of which is knownfrom, for example, U.S. Pat. No. 4,336,035.

The dust removal device illustrated in FIG. 1 is a fabric filter 10. Thefabric filter 10 comprises a housing 26. The effluent gas enters thehousing 26 via an open lower end 28 thereof. A horizontal plate 30 isarranged in an upper end of the housing 26. Extending from plate 30 area number of fabric filtering devices in the form of fabric bags 32, eachsuch fabric bag 32 extending through a corresponding opening in theplate 30. Typically, a fabric filter 10 may comprise 2 to 40 000 suchfabric bags 32. In operation, effluent gas, loaded with dust particles,including alumina, enters the open lower end 28 of the housing 26. Theeffluent gas passes through the fabric of the bags 32 and into theinterior of the bags 32, while the dust particles are collected on theoutside of the bags 32. Then, effluent gas, from which at least aportion of the dust particles have been separated, flows via theinterior of the bags 32, upward through the plate 30, and enters a cleangas plenum 34 of the fabric filter 10. Occasionally, the collected dustparticles are removed from the bags 32, for example by pulsing the bags32 with pressurized air, in accordance with the principles disclosed inU.S. Pat. No. 4,336,035, or by shaking the bags 32. The dust particlesthereby removed from the bags 32 are partly returned to the contactreactors 4, 6, and partly removed from the gas cleaning unit 1 via anoutlet 36. The removed particles would normally be directly transportedto the aluminium production electrolytic cells 20 illustrated in FIG. 2.

Continuing with the description of FIG. 1, the clean gas plenum 34,which is arranged at the top of the fabric filter 10, above the plate 30and the bags 32, is provided, in a vertical side wall 38 thereof, with ahorisontal outlet duct 40. The outlet duct 40 is fluidly connected to afan 42, which in the example of FIG. 1 is a radial fan 42. The radialfan 42 is provided with an impeller 44 arranged within a housing 45 ofthe fan 42 and rotated by a horizontal shaft 46 driven by a motor 48.The effluent gas, flowing horizontally through the outlet duct 40,enters the radial fan 42 in the axial direction of the impeller 44 andis given an impulse in a vertical direction, radially of the impeller44. The effluent gas is forwarded upwardly by fan 42 and leaves fan 42substantially vertically via fan outlet 50.

FIG. 2 illustrates, schematically, two alternative locations of a fan.In accordance with a first alternative embodiment, a fan 142 may bearranged in the gas inlet duct 2. In accordance with a secondalternative embodiment, a fan 242 may be arranged just after the wetscrubber 12. Fans 142, 242 could be utilized as alternatives to, or incombination with, fan 42 for generating a flow of effluent gas throughthe gas cleaning unit 1.

The wet scrubber 12 comprises a housing 52. The housing 52 comprises ahorizontal bottom 54, a horizontal roof 56, and a generally cylindricalside wall 58 extending between the bottom 54 and the roof 56. Thehousing 52 of the wet scrubber 12 is entirely located inside thepenthouse 14 of the gas cleaning unit 1. This means that the housing 52of the wet scrubber 12 is protected from wind loads, UV-radiation,precipitation, sand storms, etc, which substantially reduces thematerial and load requirements on the wet scrubber 12 housing 52.

The wet scrubber 12 housing 52 is arranged at a point above orvertically higher than and just beside the fabric filter 10, as is bestillustrated in FIG. 1. Upon exchanging worn or damaged filter bags 32,the filter bags 32 may be removed through vertically upward movementthereof through hatches 35 arranged in the roof 37 of the clean gasplenum 34. A filter bag 32 b, illustrated with dotted lines, indicatesthe position of the filter bag 32 b during the removal/exchange thereof.By arranging the wet scrubber 12 housing 52 beside the fabric filter 10clean gas plenum 34 the exchange of filter bags 32 is not obstructed bythe housing 52. As can be seen from FIGS. 1 and 2, the wet scrubber 12housing 52 is arranged just above the alumina silo 8.

The wet scrubber 12 bottom 54 is provided with an inlet opening 60 whichis fluidly connected to the fan outlet 50. The inlet opening 60 isfluidly connected to a gas distributor 62, which distributes the gascoming from the fan 42 inside the housing 52 of the wet scrubber 12. Anoptional horizontal gas distribution grid 64 may be arranged above thegas distributor 62 inside the housing 52 to support the formation of aneven gas distribution profile of the effluent gas inside the housing 52.Optionally, a gas-liquid contacting packing 66 may be arranged insidethe housing 52, above the gas distributor 62 and gas distribution grid64, to improve contact between effluent gas and an absorption liquidsupplied via nozzles 68 arranged inside the housing 52, above thedistributor 62, grid 64, and packing 66. Examples of such gas-liquidpacking 66 include Mellapak™ available from Sulzer Chemtech AG,Winterthur, CH, and Pall™ rings available from Raschig GmbH,Ludwigshafen, DE. In accordance with one embodiment, the gas-liquidpacking 66 may be a wooden packing made from a grid of wooden rods. Awooden packing makes it possible to operate the wet scrubber 12 withoutsupply of absorption liquid for shorter periods of time, without causingdamage to the packing material.

The absorption liquid would typically comprise water along with analkali substance. The alkali substance may for example be sodiumhydroxide, NaOH, sodium carbonate, Na₂CO₃, calcium hydroxide, CaOH,limestone, CaCO₃, or any other substance that is suitable forneutralising the acid pollutants of the effluent gas, including forexample sulphur dioxide, SO₂, and hydrogen fluoride, HF, that are to beremoved from the effluent gas by the wet scrubber 12. In accordance witha further embodiment, the absorption liquid comprising water along withan alkali substance could, at least in part, be supplied to the wetscrubber 12 in the form of seawater, for example in the form of seawaterfrom a nearby ocean. When operating the scrubber with seawater, theseawater would be passed through the wet scrubber 12 to absorb andneutralize sulphur dioxide and hydrogen fluoride from the effluent gas,after which the seawater would be returned to the ocean.

For example, the absorption and neutralisation of sulphur dioxide andhydrogen fluoride from the effluent gas using sodium hydroxide, NaOH,could occur in accordance with the following reactions:SO₂(g)+2NaOH(aq)+½O₂(g)

Na₂SO₄(aq)+H₂O  [eq. 1.1]HF(g)+NaOH(aq)

NaF(aq)+H₂O  [eq. 1.2]

A pump 70 is arranged on the ground 72 and is arranged for pumpingabsorption liquid via fluidly connected supply pipe 74 to the fluidlyconnected nozzles 68. The nozzles 68 atomize the absorption liquid andbrings it into contact, optionally with the aid of the gas-liquidcontacting packing 66, with the effluent gas flowing vertically upwardinside the housing 52 of the wet scrubber 12. The spent absorptionliquid is collected on the bottom 54 of the housing 52 and flows, via afluidly connected pipe 76, to a circulation tank 78. The circulationtank 78 is fluidly connected to the pump 70 which returns the absorptionliquid to the nozzles 68. An overflow pipe 80 is connected to the tank78 for removing excess absorption liquid.

A pH measurement device 82 is connected to the pipe 74 for measuring thepH of the absorption liquid. The pH measurement device 82 controls apump 84 which pumps an alkali solution, such as an NaOH solution, from astorage tank 86 to the pipe 74 via fluidly connected supply pipe 88. ThepH measurement device 82 controls the pump 84 to keep the pH value inthe absorption liquid supplied to the nozzles 68 via fluidly connectedpipe 74 at a predetermined value, for example at pH 6.5.

In accordance with an alternative embodiment, a pump 71 is arranged forpumping seawater, having a pH of, for example, around 7.5 to 8.5, from anearby ocean 73 to the supply pipe 74 via fluidly connected pipe 75. Theseawater is utilized as an absorption liquid in the wet scrubber 12 toabsorb and neutralize sulphur dioxide and hydrogen fluoride inaccordance with reactions that are similar to those describedhereinbefore with regard to NaOH. After such absorption andneutralisation, the seawater is returned to the ocean 73 via pipe 76 andfluidly connected pipe 77. Optionally, some fresh water, or somerecirculated seawater, may be circulated in the wet scrubber 12,together with the supply of fresh seawater from the ocean 73.

A droplet eliminator 90 is arranged vertically above the nozzles 68. Thedroplet eliminator 90 removes any droplets contained in the effluent gasbefore allowing the effluent gas to enter the stack 16. The cleanedeffluent gas, illustrated by means of an arrow AC, leaves the stack 16and is discharged into the atmosphere.

The gas cleaning unit 1 is supported on stands 92 forming together acommon support structure 94. The contact reactors 4, 6, the fabricfilter 10, and the wet scrubber 12, except ancillary equipment such aspump 70 and tank 78, together form a common stacked unit 96 which issupported by the common support structure 94 being common to the contactreactors 4, 6, the fabric filter 10, and the wet scrubber 12. In theembodiment of FIGS. 1 and 2 the alumina silo 8, the penthouse 14, andthe stack 16 also form part of the stacked unit 96, and are supported bythe common support structure 94. As is clear from a reference to FIGS. 1and 2 the entire gas cleaning unit 1 has a very small footprint with thewet scrubber 12 being arranged on a higher vertical level than thefabric filter 10, and above the alumina silo 8. Furthermore, the outletduct 40 forwarding the effluent gas from the fabric filter 10 to the wetscrubber 12 is very short, typically only 0.1 to 2 m. Still further, thestack 16 is also very short, since it is arranged directly on top of thehousing 52 of the wet scrubber 12, which is already located on aconsiderable height above the ground 72.

A method of cleaning effluent gas in the gas cleaning unit 1 involvesintroducing the effluent gas via the gas inlet duct 2. The effluent gasis brought into contact with recycled alumina particles in the firstcontact reactor 4 causing adsorption of hydrogen fluoride and sulphurdioxide on the alumina particles. A further adsorption occurs in thesecond contact reactor 6. The effluent gas is then filtered in thefabric filter 10. Such filtering causes a removal of entrained dustparticles and alumina laden with hydrogen fluoride and sulphur dioxide.The filtered effluent gas is then forwarded from the clean gas plenum 34of the fabric filter 10 and is almost immediately introduced in the wetscrubber 12 via the inlet opening 60 thereof. Inside the housing 52 ofthe wet scrubber 12 the effluent gas is brought into contact with anabsorption liquid causing a further removal of sulphur dioxide andhydrogen fluoride. The cleaned effluent gas is discharged to theatmosphere via a stack 16 arranged immediately on top of the wetscrubber 12 housing 52.

It will be appreciated that numerous variants of the embodimentsdescribed above are possible within the scope of the appended claims.

Hereinbefore, it has been described that the effluent gas enters the wetscrubber 12 housing 52 via an inlet opening 60 in the bottom 54 of thewet scrubber 12. It will be appreciated that an inlet opening may alsobe arranged in other positions at the wet scrubber 12 housing 52. Forexample, an inlet opening may be arranged in a lower portion of the wetscrubber 12 cylindrical side wall 58. Still further, an inlet openingmay be arranged in that position on the wet scrubber 12 housing 52 wherebottom 54 is joined to side wall 58.

Hereinbefore, it has been described that the wet scrubber 12 is providedwith a packing 66. It will be appreciated that the wet scrubber 12 mayalso be designed without any packing, in which case the mixing ofabsorption liquid and effluent gas relies on the atomization ofabsorption liquid by nozzles 68. An example of a useful nozzle 68 is theWhirlJet™ nozzle available from Spraying Systems Co, Wheaton, Ill., USA.It will be appreciated that nozzles 68 could be arranged in severaldifferent vertical levels inside the wet scrubber 12 housing 52.Furthermore, the nozzles 68 could be arranged to spray the liquidcounter-current, as illustrated in FIG. 1, co-current, or bothcounter-current and co-current, in relation to the flow of effluent gas.

Hereinbefore it has been described that the gas cleaning unit 1comprises a first and a second contact reactor 4, 6 in which theeffluent gas is brought into contact with alumina. It will beappreciated that a gas cleaning unit could also, in accordance with analternative embodiment, be provided with a single contact reactor, inwhich the effluent gas is brought into contact with recirculated andfresh alumina. In accordance with a further alternative embodiment, agas cleaning unit could be provided with three or more contact reactorsarranged in series.

Hereinbefore it has been described that the fan 42 is a radial fan. Itwill be appreciated that other types of fans, for example axial fans,could also be utilized for forwarding the effluent gas through the gascleaning unit 1.

Hereinbefore it has been described that the pump 70, tank 78 and pHadjustment equipment 82, 84, 86, 88 are all arranged on the ground 72.It will be appreciated that it would also be possible to arrange some orall of these devices in another location. In accordance with oneembodiment, at least one of the pump 70, the tank 78, the associatedpipes 76, 74, and the pH adjustment equipment 82, 84, 86, 88 is arrangedinside the gas cleaning unit 1 penthouse 14. In accordance with afurther embodiment, the pump 70, the tank 78, the associated pipes 76,74, and the pH adjustment equipment 82, 84, 86, 88 are all arrangedinside the penthouse 14.

Hereinbefore it has been described that the alumina silo 8 is integratedin the gas cleaning unit 1. It will be appreciated that it is alsopossible to design a gas cleaning unit having no alumina silo 8integrated therein. In such a case, fresh alumina may be supplied from aremote central alumina storage, fluidly connected to supply pipe 24.

Hereinbefore, the gas cleaning unit 1 has been described as comprisingone fabric filter 10, and one wet scrubber 12. It will be appreciatedthat a gas cleaning unit could be provided with several parallel fabricfilters, for example 2 to 100 parallel fabric filters, and a number ofparallel wet scrubbers, for example 2 to 100 parallel wet scrubbers. Thenumber of wet scrubbers need not correspond to the number of fabricfilters. Hence, for example, two parallel fabric filters could befluidly connected to one common wet scrubber.

To summarize, a gas cleaning unit 1 for cleaning an effluent gas of atleast one aluminium production electrolytic cell comprises a contactreactor 4, 6 in which the effluent gas is brought into contact withalumina, and a dust removal device 10 for removing at least a portion ofthe alumina. The gas cleaning unit 1 further comprises a wet scrubber 12in which the effluent gas is brought into contact with an absorptionliquid containing water for removing further pollutants from theeffluent gas. The wet scrubber 12 is positioned at a point verticallyhigher than that of the dust removal device 10.

While the present invention has been described with reference to anumber of preferred embodiments, it will be understood by those skilledin the art that various changes may be made and equivalents may besubstituted for elements thereof without departing from the scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from the essential scope thereof. Therefore, it isintended that the invention not be limited to the particular embodimentsdisclosed as the best mode contemplated for carrying out this invention,but that the invention will include all embodiments falling within thescope of the appended claims. Moreover, the use of the terms first,second, etc. do not denote any order or importance, but rather the termsfirst, second, etc. are used to distinguish one element from another.

The invention claimed is:
 1. A method of cleaning pollutants from aneffluent gas produced by at least one aluminium production electrolyticcell comprising: providing a gas cleaning unit; contacting within atleast one contact reactor of the gas cleaning unit the effluent gas withalumina; separating within a dust removal device of the gas cleaningunit at least a portion of the alumina having adsorbed pollutants fromthe effluent gas in the contact reactor from the effluent gas; andcontacting within a wet scrubber of the gas cleaning unit arranged at apoint vertically higher than that of the dust removal device within thegas cleaning unit effluent gas flowing vertically upwardly from the dustremoval device through a bottom of the wet scrubber with an absorptionliquid containing water for removing further pollutants from theeffluent gas.
 2. The method according to claim 1, further comprisingpassing the effluent gas from which at least a portion of the pollutantshas been separated within the dust removal device into a clean gasplenum arranged at a top of the dust removal device, for effluent gasflow horizontally out of the clean gas plenum and vertically upward flowinto the wet scrubber.
 3. The method according to claim 1, whereincontacting the effluent gas within the wet scrubber requires flow of theeffluent gas vertically upward into a housing of the wet scrubber via abottom of the wet scrubber housing.
 4. The method according to claim 1,wherein passing the effluent gas from the contact reactor to the dustremoval device is a vertically upward flow, and passing the effluent gasfrom the dust removal device to the wet scrubber is a vertically upwardflow.